12AU7 in cascode configuration

georgehifi · 17th September 2011, 05:09 AM

Hi guys what is the approx output impedance of a 12AU7 in cascode configuration. Also what is it's max current output.

Thanks Georghe

SY · 17th September 2011, 05:27 AM

Just about any cascode will have an output Z roughly equal to the plate load resistor. The max peak current out will roughly equal the idle current, but the distortion with that kind of load will be unacceptably high. In a low distortion circuit (for a 12AU7, not a linearity champion!), the max current swing should be held to about one tenth of the idle current.

DF96 · 17th September 2011, 02:38 PM

Roughly (mu^2)/gm in parallel with the anode resistor. In most cases the anode resistor will dominate. There was one celebrated GEC naval receiver which used 12AU7 cascode as the RF amplifier, but for audio there are almost certainly better ways of doing whatever it is you want to do.

kevinkr · 17th September 2011, 07:52 PM

I'd recommend a 6DJ8 in cascode over the 12AU7A for linearity and noise performance, and unlike the 12AU7A it was actually designed with cascode operation in mind.

revintage · 17th September 2011, 10:21 PM

Hey Georghe,
How much gain do you need and what is the application?

As others have told you 12AU7 is next to no, no.

Raw gain of a E88CC is a few dB over 55dB. You can easily get over 40dB gain with acceptable low THD.

If you need low Zout you can always add a follower.

georgehifi · 12:49 AM

Sorry guys I'm not making one, a tube guru just said to me yesterday this would be around 33k output impedance and maximum of 10mA current output. Just trying to find out. It says it's 12AU7 in cascode configuration, directly connected to the output rca's (probably via coupling cap) and that it has switchable gain of 6db or 12db.
Concert Fidelity | CF-080 LSX
He uses it into a power amp that is 20k input impedance, this then would be a bad impedance match.
Cheers George

revintage · 08:48 AM

As the machine is called hybrid I can guarantee it has a sand buffer with very low Zout, not 33k. So it is probably no problem for your friend.

About the circuit low gain also indicates local feedback, maybe through the upper grid and an unbypassed cathode resistor. Still one wonder why 12AU7 was choosen and what a cascode is needed for when total gain is 6/12dB?

georgehifi · 18th September 2011, 09:24 AM

Quote:

Originally Posted by revintage

As the machine is called hybrid I can guarantee it has a sand buffer with very low Zout, not 33k. So it is probably no problem for your friend. ?

But it says that the 12au7's are directly connected to the output rca's somewhere in the blurb.

Cheers George

revintage · 18th September 2011, 09:48 AM

Hey George,

Have tried to find Zout in the blurb, but nothing. But I read there is a sand input stage and the cascode at the output. So 33k Zout could be true!

Weired deLuxe!

SY · 18th September 2011, 10:33 AM

This may come as a shock, but... hope you're sitting down... is there a fainting couch nearby?...

....sometimes ad tear sheets are not technically accurate.

17th September 2011, 05:09 AM	#1
georgehifi diyAudio Member Join Date: Sep 2003 Location: Manly Australia (Jewel of the Pacific)	12AU7 in cascode configuration Hi guys what is the approx output impedance of a 12AU7 in cascode configuration. Also what is it's max current output. Thanks Georghe __________________ Avatar : Production Lightspeed Attenuator www.lightspeedattenuator.com

17th September 2011, 05:27 AM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Just about any cascode will have an output Z roughly equal to the plate load resistor. The max peak current out will roughly equal the idle current, but the distortion with that kind of load will be unacceptably high. In a low distortion circuit (for a 12AU7, not a linearity champion!), the max current swing should be held to about one tenth of the idle current. __________________ If there's a sucker born every minute, where do the rest of them come from?

17th September 2011, 07:52 PM	#4
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	I'd recommend a 6DJ8 in cascode over the 12AU7A for linearity and noise performance, and unlike the 12AU7A it was actually designed with cascode operation in mind. __________________ "Extraordinary claims require extraordinary evidence." - Carl Sagan

18th September 2011, 12:42 AM	#6
georgehifi diyAudio Member Join Date: Sep 2003 Location: Manly Australia (Jewel of the Pacific)	Sorry guys I'm not making one, a tube guru just said to me yesterday this would be around 33k output impedance and maximum of 10mA current output. Just trying to find out. It says it's 12AU7 in cascode configuration, directly connected to the output rca's (probably via coupling cap) and that it has switchable gain of 6db or 12db. Concert Fidelity \| CF-080 LSX He uses it into a power amp that is 20k input impedance, this then would be a bad impedance match. Cheers George __________________ Avatar : Production Lightspeed Attenuator www.lightspeedattenuator.com Last edited by georgehifi; 18th September 2011 at 12:49 AM.

18th September 2011, 08:41 AM	#7
revintage diyAudio Member Join Date: May 2007	As the machine is called hybrid I can guarantee it has a sand buffer with very low Zout, not 33k. So it is probably no problem for your friend. About the circuit low gain also indicates local feedback, maybe through the upper grid and an unbypassed cathode resistor. Still one wonder why 12AU7 was choosen and what a cascode is needed for when total gain is 6/12dB? Last edited by revintage; 18th September 2011 at 08:48 AM.


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

17th September 2011, 02:38 PM	#3
DF96 diyAudio Member Join Date: May 2007	Roughly (mu^2)/gm in parallel with the anode resistor. In most cases the anode resistor will dominate. There was one celebrated GEC naval receiver which used 12AU7 cascode as the RF amplifier, but for audio there are almost certainly better ways of doing whatever it is you want to do.

17th September 2011, 10:21 PM	#5
revintage diyAudio Member Join Date: May 2007	Hey Georghe, How much gain do you need and what is the application? As others have told you 12AU7 is next to no, no. Raw gain of a E88CC is a few dB over 55dB. You can easily get over 40dB gain with acceptable low THD. If you need low Zout you can always add a follower.

18th September 2011, 09:48 AM	#9
revintage diyAudio Member Join Date: May 2007	Hey George, Have tried to find Zout in the blurb, but nothing. But I read there is a sand input stage and the cascode at the output. So 33k Zout could be true! Weired deLuxe!

18th September 2011, 10:33 AM	#10
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	This may come as a shock, but... hope you're sitting down... is there a fainting couch nearby?... ....sometimes ad tear sheets are not technically accurate. __________________ If there's a sucker born every minute, where do the rest of them come from?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Cascode Tube Mic Preamp: 12AX7/12AU7 Schematic	88man	Tubes / Valves	13	4th August 2012 12:08 AM
12AU7 Amp?	thefragger	Tubes / Valves	20	8th March 2012 12:26 AM
Better than 12au7?	dgta	Tubes / Valves	13	3rd July 2011 11:32 PM
Preamp tube SRPP 12AU7 VS preamp tube Aikido 12AU7 = all use rec 6X5GT	spiritm2r	Tubes / Valves	1	6th March 2011 02:53 PM
12AT7,12AU7 Ultra linear cascode	ppl	Tubes / Valves	33	21st April 2010 06:43 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read